The main sources of hematopoietic stem cells (HSCs) are the bone marrow and the umbilical cord blood (UCB). HSCs are used in the transplantation setting (autologous or allogeneic) which constitutes one of the most effective treatment strategies for achieving cures in patients with hematologic malignancies, bone marrow failure conditions, a variety of congenital diseases of global concern (e.g. sickle cell anemia and thalassemia) and autoimmune diseases such as lupus. However, this opportunity for life-saving or life-improving treatment is not available to many thousands of people worldwide due to an inability to amplify these cells ex vivo sufficiently to make the procedure safe and successful. More particularly, for every 3 patients, one will forego the opportunity for transplant because no human leucocyte antigen (HLA) identical donor can be found. Another proportion of patients will not have access to transplantation simply because too few HSCs are available in the graft (i.e. cord blood or autologous) for successful transplant. The safety and efficacy of marrow transplant is directly dependent on the number of HSCs and HPCs available for engrafting. The more that can be infused, the more rapidly is hematologic function restored, and the shorter is the window of risk for infection due to lack of granulocytes or of bleeding due to lack of platelets. The challenge in providing sufficient HSCs is further escalated where non-myeloablative conditioning is preferred such as in the context of gene therapy for major inherited blood disorders (the major genetic cause of morbidity and mortality worldwide).
In adults, HSCs mainly reside in the bone marrow and must be mobilized to enter the circulation prior to being collected by apheresis, either for autologous or allogeneic hematopoietic stem cell transplantation (HSCT). The collection of an adequate number of CD34+ cells, a surrogate marker of (HSCs), is paramount because the dose of CD34+ cells influences the success and rate of hematopoietic recovery. Several reports suggest that a higher infused CD34+ cell dose is independently predictive of improved survival.
The two most commonly used mobilizing regimens are granulocyte-colony stimulating factor (G-CSF) and G-CSF plus chemotherapy. Plerixafor, a CXCR4 antagonist approved by the United States Food and Drug Administration (FDA) in 2008 and in 2011 by Health Canada, enhances mobilization of HSCs when administered with G-CSF. However, Plerixafor is contraindicated in patients with leukemia because of mobilization of leukemic cells. Inability to obtain sufficient numbers of CD34+ cells/kg with currently used mobilization regimens is estimated to affect up to 15% of patients (varies between diseases). Use of autologous HSCT in hematological malignancies is often limited by the fact that both normal and cancer stem cells are present in the bone marrow and thus, likely to be mobilized.
Allogeneic HSCT with BM or mPBSC is another transplantation alternative. However, about one third to one fourth of the patients who are eligible for this type of transplant cannot find a suitable donor. For those who get transplanted, there is a high frequency of transplant related mortality due to graft-versus-host disease, relapse or graft rejection; and a risk of immunodeficiency for prolonged periods of time. Alternatively, umbilical cord blood has been shown as a valid option in allogeneic HSCT. However, a single CB unit typically provides insufficient HSCs for an adult patient for a rapid and efficient hematopoietic recovery.
There is thus a need for novel strategies for increasing the expansion of hematopoietic stem cells, hematopoietic progenitor cells, or both hematopoietic stem cells and hematopoietic progenitor cells.